As you read this, your body is crackling with electricity. The 639 muscles in your body are in a state of sustained partial contraction, giving your body a kind of constant background 'static'. Your body contains three basic types of muscle: striated, smooth, and cardiac. It is electrical changes within striated ('voluntary') muscle that gives you the ability to move through and manipulate your external environment. Biochemically, it makes no difference whether you're built like Arnold Schwarzenegger or Danny DeVito, for all striated muscles work in precisely the same way.
A muscle cell at rest is characterized by a charge separation — called 'polarization' — across its membrane. A relative excess of sodium ions outside the membrane results in a net positive charge, while an excess of potassium ions inside results in a net negative charge. Whenever you choose to move, your central nervous system sends volleys of nerve impulses to the relevant muscle group. A flood of the neurotransmitter acetylcholine from motor neurons stimulates the breakdown of energy-storing molecules, either ATP in an oxygen-rich environment or glycogen in an oxygen-poor one. This breakdown powers a localized reversal in the distribution of sodium and potassium ions across the muscle cell membrane. This activates yet another system, causing the release of calcium ions from a system of storage membranes, which — in turn — induces the uncovering of active sites on ratchet-like proteins called 'actin filaments'. Waves of this localized charge reversal — called 'depolarization' — flow from one end of the muscle fibre to the other, stimulating contractile proteins called 'myosin fibrils' to contract and slide over the actin filaments. The myosin fibrils become short and thick, catching on molecular 'hooks' on the thin actin filaments, holding the contracted muscle taught. At the end of muscle contraction, release of a compound called creatine phosphate causes the actin hooks to relax their grip on the myosin fibrils, calcium ions are actively pumped back into their storage membranes, and the cell returns to its polarized state.
This elegant biochemical choreography holds true for all vertebrates. As a result, all backboned animals produce weak electrical currents. But some fishes have 're-wired' regions of their nerves and muscles in such a way that enables them to generate very powerful electric charges. These so-called 'electrogenic fishes' vary widely in generating capacity, form, and ecology, but are remarkably similar in their electricity producing mechanism. With the exception of the freshwater apteronotid knifefishes (in which the electric organs are derived from modified nerve fibres), the electric organs of all electrogenic fishes are modified from striated muscle fibres, consisting of stacks of flattened cells innervated on one side. This serial arrangement sums the small electric potentials arising from membrane depolarizations, thus giving rise to much higher external potentials. These 'supercharged' electrical potentials are put to uses other than locomotion. Weakly electrogenic fishes that live in very turbid water have been shown to use distortions in their electromagnetic field to navigate around obstacles and detect other organisms, including potential mates (males and females of the same species have slightly different 'signature crackles'). Strongly electrogenic fishes use their shocking abilities to stun prey or deter predators. In a few of these fishes, the electric organs can generate very large potentials — 50 to 500 volts — sufficient to modify the hairstyle or be downright dangerous to humans.
Electrogenic fishes include both fresh and salt water groups. Freshwater electrogenic fishes include the electric eel (Electrophorus), electric catfish (Melapterurus), nakedback knifefishes (Gymnotus), the African knifefish (Gymnarchus), and the elephantfishes (family Mormyridae). Saltwater electrogenic fishes include four families of rays — torpedo rays (Torpedinidae), short-tailed electric rays (Hypnidae), electric rays (Narcinidae), and shortnose electric rays (Narkidae) — certain skates (family Rajidae), as well as at least one teleost fish, the stargazer (Astroscopus). Since the freshwater electrogenic fishes are rather limited in distribution (certain rivers in South America, Central America, and Africa) and inhabit muddy waters with poor visibility, very few divers are likely to see them. Therefore, the remainder of this essay will concentrate on marine electrogenic fishes — particularly the rays — which are much more likely to be encountered by recreational divers.
Collectively, the electrogenic rays are among the weirdest of marine organisms. These rays are goofy-looking critters: pancakes with smallish pop-eyes mounted close together on the top of the head and punctuated by prominent spiracles (accessory respiratory openings), looking like comical eyebrows. Electrogenic rays are sluggish, feeble swimmers that spend most of their daylight hours lying on the bottom at favored sites partially buried in the mud or sand, generally preferring the shallows; at night, they actively forage over nearby mud or sand bottoms, kelp forest floors, or over reef faces. All electrogenic rays are ovoviviparous, the embryos wrapped in thin membranes and retained by the mother ray to be born alive after a gestation period of eight to ten months; at a late stage of development, the young are nourished by histotroph, a protein-rich liquid secreted from the mother ray's uterine lining — this womb service is sometimes known by the more descriptive term 'uterine milk'.
All electrogenic rays are classified within the order Torpediniformes, containing four families, nine genera and between 37 and 40 species. The family Torpedindae is composed of a single genus (Torpedo) and some 14 to 17 species; torpedo rays are characterized by a round or oval flabby body disc with a straight or slightly notched anterior margin, two dorsal fins, slender jaws lacking labial cartilages, and a short tail with a well developed caudal (tail) fin. The family Hypnidae contains a single species (Hypnos monopterygium) from Australian waters; the short-tailed electric ray is characterized by a vestigial tail, small dorsal and caudal fins, tricuspid teeth, and a flabby pear-shaped disc. The family Narcinidae includes four genera (Benthobatis, Diplobatus, Discopyge, and Narcine) and approximately 18 species; electric rays are characterized by oval discs that are usually longer than they are wide, the anterior edge of which is broad and rounded, two dorsal fins, a well-developed caudal fin, lateral folds on the tail, stout jaws and strong labial cartilages. Lastly, the family Narkidae contains three genera (Narke and Typholonake with one dorsal fin, and Temera with none) and four species; the shortnose electric rays are characterized by a rounded disc, a short, thick tail with a large caudal fin, a shallow groove around the mouth, short transverse jaws, and a short snout.
The electric organs of torpedinoid rays — which typically constitute about one-sixth the total body weight of the ray — are situated on either side of the forward part of the body disc between the anterior extension of the pectoral fin and the head, extending from about the level of the eye backward past the gill region. Usually, outlines of these organs are visible externally on both the dorsal and ventral surfaces. The organs are kidney-shaped, modified from branchial (gill) musculature, and are composed of columnar prism-like structures — called 'electroplaques' — separated by loose connective tissue, forming a network similar to the cells of a honeycomb. Some species of torpedinoid ray have their electrogenic organs 'wired' to deliver electric shocks upward through the back ; presumably, this arrangement is useful for defense. Other species are 'wired' to direct shocks downward under the belly, presumably useful for stunning prey. The number of electroplaques per column — and thus the 'shock value' — also varies in number among species of electrogenic ray, from roughly 500 in Narcine to over 1 000 in Torpedo. An electrogenic ray can deliver a successive series of discharges, but becomes progressively weaker until it is finally exhausted. The voltage delivered varies with the size of individual and species, but ranges from 8 to 220 volts. Completion of the circuit by contacting the ray at two points is not necessary if the ray is in water. A single 'zap' from one of these rays is usually enough to discourage all but the most persistent of predators. Contact with a large ray may result in a shock sufficient to knock over and temporarily disable a man. People who have 'stumbled onto' these animals while wading have reported a shock similar to being hit by a very large fist. Recovery is usually uneventful, but it should be borne in mind that such electrical discharges could knock a diver unconscious.
Recent field research carried out by Chris Lowe, Dick Bray, and Don Nelson off southern California has revealed that the Pacific Torpedo Ray (Torpedo californica) generates two distinct types of electrical pulse and uses several strategies to capture prey. Using a specially-constructed electrode prod, Lowe and his co-workers discovered that the Pacific Torpedo produces regular pulses of electricity when faced with persistent researchers (and presumably other perceived threats), but that this discharge changes dramatically to intense blasts of electricity when the test subject is presented with a live fish or an electrode simulating the bioelectricity of live prey. It had been known for some time that the Pacific Torpedo is a predator of relatively large bony fishes — including anchovies, herring, kelp bass, and in one case a 120-centimetre specimen captured off California was found to contain a 60-centimetre Silver Salmon (Oncorhynchus kisutch). But until Lowe, Bray, and Nelson investigated predatory behaviour in free-ranging Pacific Torpedos, it was unclear exactly how these slow-moving predators managed to capture such fast-swimming prey. Thanks to Lowe and his co-workers, we now know that the Pacific Torpedo hunts most actively at night and uses at least three strategies to capture prey. Lying just beneath the mud or sand surface, a Pacific Torpedo lunges from the substrate when a suitable victim comes within striking range; most Torpedos emit a powerful electrical discharge to immobilize or disorient their prey. In another technique, a Pacific Torpedo Ray slowly meanders over an unaware fish, blasts it with an electric jolt, then wraps the prey with its pectoral fins and completes the maneuver with a neatly executed barrel roll to manipulate the prey into its mouth. In yet another technique, Pacific Torpedos stalk their quarry by slowly drifting over the reef or by creeping along the sea floor, taking advantage of natural cover like a trench-coated spy in a classic movie. Small wonder the nocturnal 'stalk and shock' technique of the Pacific Torpedo Ray has earned it the nickname 'night shocker'!
Recent work by P. Lyons on two western Atlantic skates (the Little Skate, Raja erinacea and the Winter Skate, R. ocellata) has indicated that these species have weak electrogenic organs in the caudal peduncle (tail stalk). The electroplaques of these skates are small disc- or cup-shaped cells that are less regularly oriented to one another than those of the electrogenic rays. These skate electric organs are organized axially, running in columns from front to back. The electric discharge in these skates is also distinctly different from that of other electrogenic fishes, either elasmobranch or teleost: the discharge has a slow time course, rising and falling over an interval measured in seconds rather than the rapid discharge that occurs in milliseconds in other electrogenic fishes. The discharges generated by Little and Winter Skates typically measure from a few millivolts to a volt or two. This voltage is intermediate between those generated by electrolocating and prey-stunning electrogenic fishes. So what — if anything — do these skates use their electric organs for? I would suggest that the weakly electrogenic organs in the tail of these and certain other skates are an anti-predator device: producing a kind of sparkler effect that 'jams' the electrosensitive organs of one of their primary predators, the angel shark (Squatina), confusing the predator long enough for an imperiled skate to save its tail.
Perhaps the most intriguing of electrogenic fishes, however, are the stargazers (family Uranoscopidae, with 9 genera and about 25 species). Stargazers have unique electric organs, modified from their eye muscles, that present diving naturalists with a very curious puzzle. These bottom-dwelling teleosts are named for their large, upward-looking eyes. Stargazers are ambush predators, lying quietly on the bottom until a potential prey fish blunders within striking range then using the short, broad tail to launch a lightning-fast 'surprise' attack from close range. Like anglerfishes (to which they are only distantly related), some stargazers rely on a small, worm-like filament extending from the floor of the mouth to lure prey fish close to their large jaws. As the jaws are distended to hoover up the prey — an action that takes only 150 to 300 milliseconds — the stargazers electric organs fire a burst of pulses at high frequency, followed by a train of pulses lasting a second or so. Work by P.E. Pickens and W.N. McFarland on Astroscopus has revealed that the discharges are less than a volt (measured at the mouth) and insufficient to stun prey. Remarkably, the duration of the electrical burst which occurs as the mouth is opened correlates with the length of the prey fish. It is almost as though the discharge signals to other stargazers the pride that the captor feels in the size of its meal! Whatever its real significance, the electric discharge of stargazers is certainly not used for stunning prey or electrolocation and the work of Pickens and McFarland provides us with an amusing possibility.
Perhaps if the stargazer can 'announce' the size of its prey, maybe it can also 'lie'. Fishermen are notorious for exaggerating the size of 'The One that Got Away'. Who knows? Perhaps you are the subject of one of these 'fish stories', the stuff of which stargazer legends are made: "Oh you should have seen him! He was magnificent, black neoprene, blue mask and flippers, he was two hundred pounds if he was a ounce >sigh< — I just can't talk about it right now ... "
— R. Aidan Martin